The aryl hydrocarbon receptor (AHR) is a transcription factor that without ligand exists in the inactive state in the cytoplasm bound to HSP90. Upon ligand binding, AHR translocates to the nucleus where it dimerizes with ARNT forming a functional transcription factor. AHR/ARNT binds dioxin response elements (DRE) in the promotor of many genes where it modulates gene transcription. The most well documented genes regulated by AHR are the cytochrome P450 genes Cyp1b1 and Cyp1a1, where activation of AHR greatly increases expression of these genes. Therefore, Cyp1b1 and Cyp1a1 mRNA levels are a selective readout of AHR activation (reviewed in Murray et al., 2014).
Many exogenous and endogenous agonists of AHR exist that activate the receptor. The best characterized exogenous ligand class are the dioxins. One of the first endogenous ligands to be characterized is kynurenine, generated by TDO (Opitz 2011) or IDO (Mezrich 2010). Kynurenine is a stable metabolite in the IDO/TDO pathway and is the product of tryptophan degradation. Kynurenine has been shown to activate AHR as measured by an increase in Cyp1a1 and/or Cyp1b1 mRNA levels in multiple cell types, along with other DRE-driven genes.
AHR activation has pro-tumor effects by acting directly on the tumor cells and indirectly by causing immunosuppression, therefore not allowing the body's own immune system to attack the tumor. For example, AHR activation through multiple ligands leads to increased expression of FoxP3 and results in a polarization of CD4+ T-cells toward a suppressive subset called Foxp3+T-regulatory cells (Tregs). These T-reg cells inhibit the proliferation of activated T cells (Funatake 2005, other refs). Interestingly, kynurenine has been shown to induce immunosuppressive Tregs through AHR. Kynurenine does not affect T-reg generation in AHR-null T cells or when an AHR antagonist is added (Mezrich). In addition to T-regs, AHR activation also leads to expansion of suppressive Tr1 T cells (Gandhi 2010). It has also been shown that expression of IDO is regulated by AHR activation in both tumor cells and T cells, leading to increased immune suppression (Vogel). It is likely there is also a role for AHR in immune suppressive myeloid cells (Nguyen 2013). Immune suppression is often associated with high levels of anti-inflammatory cytokines and there is evidence that AHR is involved in activation of many of these cytokines, such as IL-10 (Gandhi 2010, Wagage 2014).
There remains an unmet need to develop inhibitors of AHR for treating diseases, disorders and conditions associated therewith.